This invention relates to electrostatic spray systems and, more particularly, to a nozzle assembly for electrostatic spray guns. More particularly, this invention relates to an external air atomizing and fan-shaping nozzle assembly for electrostatic spray guns useful in applications involving relatively low liquid coating material flow rates.
In conventional electrostatic spray systems, fluid coating material such as paint, varnish, lacquer and the like is passed through the barrel of a spray gun, into a fluid tip which is threaded at its rear into a counterbore in the forward end of the barrel and through and out of a small diameter orifice at the forward end of the fluid tip. An air cap surrounds the forward end of the fluid tip and includes a central bore surrounding the fluid tip so as to define an annular air passage around the fluid tip orifice. Air issuing from this annular passage impacts with the stream of material issuing from the material orifice of the fluid tip to at least coarsely atomize the material stream. There may be additional openings or ports in the air cap to further atomize the liquid coating material. The air cap includes a pair of opposed air horns which include pairs of ports through which pressurized air likewise passes. This air is used to change the conical shaped atomized spray of material issuing from the fluid tip into a flat fan for better coverage of the part to be coated. A trigger operated valve controls the flow of air through the atomizing air passage and a manually adjustable valve controls the amount of air issuing from the horns of the nozzle and thus the degree of fan formed by the atomized spray.
In such systems, it is generally deemed important that both the atomizing air and the fan-shaping air impact the material uniformly, i.e., uniformly around the fluid tip in the first case and uniformly from the opposed air horns in the second case. That is, to form the desired flat fan spray pattern which is uniform in shape, it is important that the flow of air issuing from the air horn ports be equal on both sides. If the flow of air is not equal out of the air horns, the fan pattern will be skewed or asymmetrical. In known electrostatic spray systems, the pressurized air used for forming the fan enters an internal chamber surrounding the fluid tip and passes through passageways in the air horns and out like pairs of exit ports. Because there is typically but one air passageway opening into the internal chamber, the pressurized air must uniformly distribute itself throughout the chamber before entering the air horns so that equal flows of fan-forming air will issue from the opposed horns. By virtue of the way the air cap is mounted to the gun barrel, it is not uncommon for the passageway of one of the air horns be closer to alignment with the air passageway passing through the barrel and opening into the internal air chamber such that an unbalanced air flow occurs between the air horns. Although this may cause some distortion of the fan, in applications where material flow rates were relatively high it is not deemed to materially adversely affect the operation of the gun.
However, more recently, the transfer efficiency of electrostatic spray systems has increased to an extent that it can reduce the amount of paint needed to cover a given surface up to 80%. This corresponds to an increase in transfer efficiency on the order of 400%. Moreover, the solids content of the paints used today has increased on the order of 200% over that in the past. As a result of the higher efficiency and increased solids content of the paint, the liquid coating material flow rates through the fluid tip have been cut by a factor of 8. With these low flows, which are on the order of less than about 3 fluid ounces of liquid coating material per minute producing fans up to 15 to 22 inches in width at a 14 inch nozzle-to-workpiece distance, the formation of the fan has become critical, and it has been found that very small differences in air flow through the fan-shaping horns seriously affects the fan pattern. Thus, when one of the horns is more closely aligned with the air flow passageway through the barrel, more air can flow through that horn than its opposed horn resulting in unbalanced air flow to the horns and a skewed or asymmetrical fan pattern.
Thus, the problem of controlling the shape of the atomized spray emitted from the gun greatly increases as the flow rate of liquid coating material through the gun decreases. In sum, very small variations in the air flow through the fan-shaping horns have been found to have very drastic effects on the shape of the pattern emitted from the gun.